* Before Heating: Copper sulfate crystals are typically a bright blue color. This blue color comes from the hydrated copper sulfate molecules (CuSO₄·5H₂O).

* After Heating: When heated, copper sulfate crystals lose their water of hydration and turn white. The chemical reaction looks like this:

CuSO₄·5H₂O (blue) → CuSO₄ (white) + 5H₂O (water vapor)

Important Note: The white anhydrous copper sulfate (CuSO₄) can readily reabsorb water from the air and turn blue again if left exposed. This is why copper sulfate is often used as a desiccant (drying agent).